The investigation will focus on details of the photochemistry of the light response of bacteriorhodopsin in the purple membrane of Halobacterium halobium as a model for rhodopsin in vertebrate rod membranes. Thermochemistry of the retinal aldimine linkage to bacterioopsin and of the photochemical cycle will be investigated by photocalorimetry. Influence of bacteriorhodopsin microenvironment will be probed by comparative study of phototransient kinetics by modulation excitation spectroscopy of native purple membrane and purple membrane in which the native lipids have been replaced.